feat(nightlies): Add ability to build from nightlies (#211)

This patch adds the optional IMAGE env variable to set the base
image and adds nightly builds to Travis CI.

.env.example

@@ -1,3 +1,4 @@
+IMAGE=sentry:9.1
 # Run `docker-compose run web config generate-secret-key`
 # to get the SENTRY_SECRET_KEY value.
-SENTRY_SECRET_KEY=
+SENTRY_SECRET_KEY=